Transmission devices known from practice, such as automatic transmission devices, typically feature a hydraulic system that is supplied with hydraulic fluid by a pump device. As is generally known, pump devices include, among other things, so-called fixed-displacement pumps, the flow rate of which depends on the drive speed, or variable-displacement pump devices, the flow volume of which is adjustable. Pump devices are usually positioned in the area of transmission input shafts and are driven by drive units of vehicle drive trains of vehicles designed with automatic transmissions, whereas requirements that vary greatly are generally to be covered through such pump devices.
Many hydraulic actuators of such hydraulic systems, such as clutches or shift rods, are of a nature such that are supplied with a defined volume of oil or a defined level of pressure for a desired operation. Particularly for transmission devices designed with wet dual-clutch systems, through the pump devices, hydraulic resisters, such as cooling oil and lubricant oil connections for the dual-clutch system, for the wheel set of the transmission device and for bearing devices, are supplied with hydraulic fluid.
The last-mentioned hydraulic resistors or actuators often operate at pressure levels that are much lower than other hydraulic consumers of the transmission devices, such as dual clutches or shifting actuators. The maximum pressure level to be available is a determining factor for the hydraulic power draw of a pump device. If all consumers are supplied through a single pump, for the supply of cooling lines and lubricant oil lines, clearly more power must be made available by internal combustion engines. It is favorable for energy purposes if each of such consumers, which preferably is able to be operated at a similarly high pressure level, is supplied with hydraulic fluid by a shared pump unit, and the transmission device is designed with several pump units. However, purely from an energy perspective, this increases the installation space and the production costs of a transmission device to an undesired extent.
Since, viewed in terms of energy, a drive unit of a vehicle drive train designed as an internal combustion engine represents the largest source of loss of a vehicle, the greatest energy savings are achievable by switching off the internal combustion engine. With newer generations of vehicles that are designed with start-stop systems, for example during a vehicle standstill or during a so-called “coasting mode,” internal combustion engines are switched off.
However, disadvantageously, the pump devices for the transmission devices described above are not driven in the switched-off operating state of the drive units, which is why hydraulic sub-systems of a hydraulically actuated automatic transmission then idle, at least in part. If, upon a subsequent restart of the internal combustion engine in the area of a transmission device, a requested operating state is to be presented, but is only possible through a corresponding functioning hydraulic system, i.e., through a fully filled hydraulic system, the transmission device is only available again with the desired functionality after the expiration of an undesired downtime that impairs the spontaneity of a vehicle. Such downtimes are not accepted by drivers, and therefore must be avoided.
Furthermore, at higher vehicle speeds, a lack of hydraulic supply of the wheel set of a transmission device leads to an insufficient lubrication or injection lubrication, which, during unfavorable operating conditions, causes loads that are not negligible in the area of the wheel set.
With transmission devices designed as dual-clutch systems, during a coasting mode, there is the option of having a gear engaged in the transmission itself, in order to, for example, realize the reconnection of the internal combustion engine during or after the restart of the internal combustion engine. However, in the switched-off operating state of a drive unit of a vehicle drive train, during which the pump device does not advance any hydraulic fluid, the actuation of the actuators of the hydraulic system of the transmission device, and thus gear changes, are no longer possible, by which time periods, through which a coasting mode can be realized, are limited to an undesired extent. This limitation is particularly pronounced when coasting or sliding, starting from higher vehicle speeds.
In order to supply a hydraulic system with hydraulic fluid, for example, during a coasting mode or during a switched-off operating state of the drive unit of a vehicle drive train to a desired extent, as is generally known, transmission devices feature an additional electrically drivable pump for hydraulic supply. However, such additional electrically drivable pumps are characterized by an unfavorable relationship between the hydraulic power that can be made available and the necessary need for installation space. In addition, electrically drivable pumps are operable only with efficiencies that are unfavorable in terms of energy.
EP 2 336 590 A2 discloses a pump unit for a dual-clutch transmission of a motor vehicle. The pump unit comprises a pump that generates, in a first operating state, a cooling flow for clutches, in a second operating state, a power flow for control elements of the dual-clutch transmission, and in a third operating state, both the cooling flow and the power flow. The pump is designed as a screw-spindle pump, and is driven by the engine of a vehicle drive train.
However, it is disadvantageous that, during a coasting mode of a vehicle designed with the pump unit, during which the drive unit is switched off, the hydraulic system of the dual-clutch transmission is not able to be supplied with hydraulic fluid to a desired extent.
Furthermore, DE 10 2005 042 685 A1 discloses a transmission with two transmission oil pumps, with which a first transmission pump is driven on the side of the drive unit, and makes available a pressure level depending on the drive speed and a volume flow of hydraulic fluid depending on the drive speed. This volume flow of hydraulic fluid serves as the drive for a second transmission pump designed as a suction jet pump, through which a volume flow of hydraulic fluid is increased for a secondary pressure circuit.
However, with a stationary drive unit, neither the secondary pressure circuit nor a primary pressure circuit of the transmission is able to be supplied with hydraulic fluid through the two transmission oil pumps.